1. Field of the Invention
The present invention relates to a radial pneumatic vehicle tire for which tread surface anomalies causing user dissatisfaction are diminished without decrease in tire performance such as wet traction and braking performance. More specifically, the invention relates to a pneumatic tire having a plurality of axially spaced apart essentially longitudinal grooves separating essentially longitudinal ribs. On at least one of said ribs, transverse grooves or cuts repeat in the circumferential direction to form first and second land portions wherein the first land portions comprise blocks having a circumferential length greater than that of the second land portions.
2. Description of Related Art
In order to improve the wet traction, wet grip, braking performance and the like, radial pneumatic tires have treads with longitudinal or zigzag grooves extending in the circumferential direction, and, for further traction improvement, lateral grooves axially connecting the circumferential grooves to form blocks. To maintain a good level of traction performance, the lateral grooves or cuts need to be present throughout the service life of the tire tread. Unfortunately, to achieve this the tire must have lateral grooves whose depth is substantially equal to the depth of the longitudinal grooves. An example of such a prior art tire 100 is shown in FIGS. 1a and 1b, respectively, in a full tire view and a plan view of the tread portion of the tire. In this example the tread blocks 20 are circumferentially spaced apart by the substantially full depth lateral grooves 30. Tire treads so designed are commonly used on the drive axle of vehicles and have acceptable wet traction performance, but are known to have reduced tread rigidity resulting in the formation of tread surface anomalies such as a "heel-and-toe" or "sawtooth" profile or tread block depression. These anomalies result in user dissatisfaction due to either unacceptable visual appearance of the tire or ride discomfort caused by tread induced vibrations. Either factor can cause removal of the tire from service prior to delivering its full potential tread service to the user.
To achieve some kind of compromise between surface anomalies and traction performance, tires have been designed having lateral grooves defining blocks 20 where the lateral grooves 30 have a depth d substantially less than the depth h of the longitudinal grooves, an example of which is tire 200 shown in FIG. 2a. The land portions of the tread bounded by edge 22 of a first block 20 and by edge 21 of a second block 20 are commonly referred to as "bridges". For values of d/h near zero, tires will have poor traction, and for values of d/h approaching unity, tires may develop surface anomalies leading to reduced service life of the original tread. An acceptable result can be obtained when the tire tread is designed so that the ratio R.sub.1 =d/h of groove depth d to the tread depth h is such that d/h is between about 0.1 to about 0.2. Unfortunately, tires experience a loss of tread rubber due to factors such as abrasion, fatigue and the like during their service lives. As a result, tires having tread designs such as shown in FIG. 2a, that is with shallow transverse grooves, will wear in such a manner that the ratio d/h will continually decrease and eventually approach a value of zero. The disadvantage of such a tire wherein d/h approaches zero is the aforementioned loss of wet grip, braking performance and the like.
Tests under highway use conditions were conducted on tires such as tire 200 having a new tire tread depth of approximately 20.5 mm with lateral grooves approximately 3 mm deep. The evolution of d/h just described is demonstrated by the test results shown in FIG. 2b which shows the measured tread depth versus circumferential position for a section of the tire. After 54,000 kilometers of service the tread depth has reached an approximate value of 17 mm everywhere, and the ratio of d/h is approximately zero. In this case the tires are more often removed from service for a perceived loss of traction rather than for the onset of surface anomalies. In an effort to mitigate this counterperformance, tire designers often add additional siping or employ complex block geometry which, instead of improving the situation, may further generate surface anomalies and/or sensitivity to chipping or tearing. Thus a tire tread design that maintains the optimum value of the ratio d/h throughout the service life of the tread is needed.